Light emitting devices generally comprise an active light-emitting layer sandwiched between an n-type and p-type semiconductor layer, with the generated light being emitted through one of these semiconductor layers. When the light strikes the light-exit surface of the semiconductor surface, however, some of the light is reflected back toward the active layer due to the difference of refractive indexes between the light-exit surface and the air or other medium through which the light is emitted (‘internal reflection’). Some portion of the reflected light may subsequently be able to exit the light-exit surface, but some portion will be absorbed in the materials within the light emitting device.
To reduce the amount of light that is internally reflected, thereby increasing the amount of light that is extracted from the light emitting device, the light-exit surface is purposely roughened. The non-planar surface increases the likelihood that the light from the active layer, which is propagated in a variety of directions from the active layer, will strike some feature of the roughened surface that allows the light to escape from the surface.
FIGS. 1A-1D illustrate a conventional process for creating a light emitting device 100 with increased light extraction efficiency.
FIG. 1A illustrates creation of the semiconductor device on a growth substrate 110. An n-type layer 120 is grown on the growth substrate 110, followed by an active layer 130, and a p-type layer 140. Contact pads 150 are provided for external contact to the n-type and p-type layers; insulating and internal components to provide this coupling are not shown, for ease of illustration. In like manner, the layers 120, 130, 140 may comprise multiple layers of materials, and other layers or vias may also be present. In an alternative arrangement, a p-type layer 140 may be grown on the substrate 110, followed by active layer 130 and n-type layer 120.
Because the contact pads 150 are typically opaque, the light emitted from the active layer 130 is extracted from the surface opposite the contact pads 150. If the growth substrate 110 is transparent it may be left intact. Otherwise in order to avoid absorption of the emitted light or to add additional scattering to the structure, the growth substrate 110 is removed to form a thin film device, and the light is emitted from the n-type layer 120. FIG. 1B illustrates the conventional orientation of an illustration of a light emitting device 100 after removal of the substrate 110, with the contact pads 150 on the ‘bottom’ of the device 100 and the light-exiting layer 120 on the ‘top’ of the device 100, commonly known in the art as a “Flip-Chip” embodiment.
As noted above, to increase the amount of light that is able to escape from the light-exiting surface 125 of layer 120 compared to the amount of light that is internally reflected and absorbed (the ‘light extraction efficiency’), the light emitting surface 125 is roughened. A number of techniques are available for roughening the surface 125, two common techniques being photo-electrochemical (PEC) wet etching and photochemical (PC) wet etching.
As described in MICROMACHINING OF GaN USING HOTOELECTROCHEMICAL ETCHING, A PhD Dissertation submitted to the Graduate School of the University of Notre Dame, by Bo Yang, Patrick Fay, Director, Graduate Program in Electrical Engineering, April 2005, the light from a high intensity source is absorbed in the semiconductor layers near or at the semiconductor-electrolyte interface. The holes that are generated drift under the influence of valence band bending towards the interface. There the holes represent broken crystal bonds and enable etching that would not occur without illumination. The roughness of the etch results from the uneven distribution of holes on the surface, leading to an uneven local etch rate. The material properties affect the etching results significantly. For example, as described in Section 2.3.5 and references therein, the density of topographical features is directly related to the dislocation density in the material. Layers of higher defect potential such as AlGaN compared to GaN will have a higher density of features. As a second example, by filtering the spectrum of the high intensity source, the relative etch rates of two materials of different bandgap may be modified. Finally, by adjusting the light intensity and molarity, the relative etch rates of layers of different defect density may be affected.
FIG. 1C provides a conceptual illustration of the result of a roughening process on the light-exiting surface 125 of a light emitting device 100, and FIG. 1D provides an image of an actual surface of a conventional LED that has been roughened by PEC etching. As illustrated, the roughening process produces a fairly random three-dimensional topology, the topology being dependent upon the composition of the material of the layer 120 being etched as well as the parameters of the roughening process, such as the concentration and type of etchants used, the temperature and duration of etching, applied electrical bias and so on. Conventionally, different sets of etching process parameters are tested with the particular material to be etched to determine the set that provides the best light extraction efficiency for that material. The determined best set is subsequently used for producing LEDs that use this material.